Differences in carbon isotope leaf‐to‐phloem fractionation and mixing patterns along a vertical gradient in mature European beech and Douglas fir

Summary While photosynthetic isotope discrimination is well understood, the postphotosynthetic and transport‐related fractionation mechanisms that influence phloem and subsequently tree ring δ 13 C are less investigated and may vary among species. We studied the seasonal and diel courses of leaf‐to‐phloem δ 13 C differences of water‐soluble organic matter ( WSOM ) in vertical crown gradients and followed the assimilate transport via the branches to the trunk phloem at breast height in European beech ( Fagus sylvatica ) and Douglas fir ( Pseudotsuga menziesii ). δ 13 C of individual sugars and cyclitols from a subsample was determined by compound‐specific isotope analysis. In beech, leaf‐to‐phloem δ 13 C differences in WSOM increased with height and were partly caused by biochemical isotope fractionation between leaf compounds. 13 C‐Enrichment of phloem sugars relative to leaf sucrose implies an additional isotope fractionation mechanism related to leaf assimilate export. In Douglas fir, leaf‐to‐phloem δ 13 C differences were much smaller and isotopically invariant pinitol strongly influenced leaf and phloem WSOM . Trunk phloem WSOM at breast height reflected canopy‐integrated δ 13 C in beech but not in Douglas fir. Our results demonstrate that leaf‐to‐phloem isotope fractionation and δ 13 C mixing patterns along vertical gradients can differ between tree species. These effects have to be considered for functional interpretations of trunk phloem and tree ring δ 13 C.